Cleaning of filters

ABSTRACT

A filter (10) has a shell (11) within which there is a bundle of microporous fibres (12). Pressurized feed suspension is introduced through inlet (15) and passes over the external walls of the fibers (12) with the clarified liquid being drawn from the lumens of the fibres (12) through outlet port (16) and the concentrated ffee being discharged through outlet (17). The solids retained within the shell are removed by a gaseous backwash cycle in which pressurized gas is introduced through the lumens and passes through the wall of the fibres (12) to dislodge solids retained on or in the fibre walls. The gaseous cleaning step is enhanced by varying the pressure within the housing (11) of the filter (10) while the gas is being introduced into the filter.

FIELD OF INVENTION

This invention relates to the cleaning and removal of solids frommicroporous hollow fibre filters.

BACKGROUND ART

International Patent Application Nos. PCT/AU84/00192 and PCT/AU86/00049describe methods for backwashing elastic microporous hollow fibrefilters The filters disclosed in these applications consist of a bundleof polymeric (such as polypropylene) fibres contained within a housinghaving a feedstock inlet thereto and a concentrate outlet therefrom. Thefeedstock is applied to the outside of the fibres and some of the liquidcontained in the feedstock passes through the walls of the fibres and isdrawn off from the fibre lumens as filtrate.

The fibres are cast in resin at both ends of the shell or housing withthe ends of the lumens open to constitute a tube-in-shell configuration.Although not described in our above mentioned specifications, the fibresmay be cast into one end of the housing with the other ends of fibresfree but with the free ends of the lumens sealed to constitute acandle-in-shell configuration.

During the filtering operation, which may be either to recover clarifiedliquid or to recover concentrated solids, solids contained in thefeedstock either pass out of the shell with the remainder of thefeedstock carrier stream, or are retained on or in the fibres. Theseretained solids cause fouling and blockage of the filter. Industrialpractice with the more common tube-in-shell microfilters for many yearswas commonly to apply the feedstock to the inner surface of the fibresby forcing flow through the fibre lumens at such a rate that turbulencescoured the walls of the fibres, retarding blockage by solid material.

In the above mentioned specifications, the feedstock is applied to theouter surface of the fibres, with a penalty of low feedstock flowvelocity and consequent low turbulence resulting in a rapid rate ofblockage of the pores of the fibres. This is overcome by the applicationof a two-stage backwashing cycle.

In the first stage a liquid backwash is applied to the lumens of thefibres such that the liquid passes through the porous walls of thefibres and sweeps retained solids out of substantially all of the poresin the walls of the fibres. In the second stage, a gaseous backwash isapplied to the lumens of the fibres such that the gas passes through thelarger pores in the walls of the fibres, stretching them and dislodgingretained blocking solids.

International Patent Application No. PCT/AU86/00049 discloses a methodof applying pressure such that the gaseous backwash is applied evenlyover the inner surface of the hollow fibres. In this method, the volumeof liquid backwash is that volume of liquid trapped in the pores of thewalls of the fibres. When the backwash stage is begun, low pressure gasis applied to clear the fibre lumens of liquid, and then high pressuregas is applied so as to exceed the bubble point of the fibres and forcegas through the larger pores in the fibre walls.

The application of the two-stage backwashing regime discussed aboverestores filtrate flux to a high value that is, however, not as high asthe initial value At each stage this slight diminution of flux reducesthe filtration capacity of the fibres. Eventually chemical cleaning isrequired. This is expensive and time consuming

Another method of cleaning the fibres is known as reverse flow and isreviewed in "Ultrafiltration Membranes and Application", Edited by A.R.Cooper, a record of a Symposium of the American Chemical Society, Sept.11-13, 1979, Pages 109 to 127, "Advances in Hollow Fibre UltrafiltrationTechnology", by B.R. Breslau.

In the Breslau method the feed is applied to the lumens of the fibres athigh velocity so that there is a large pressure drop down the length ofthe fibres By closing off the filtrate flow at the distal end of theshell, the filtrate pressure climbs within the shell and forces filtratebackwards through the fibre walls in the distal end of the fibre bundle.The direction of flow of feedstock is then reversed and the processrepeated so as to force filtrate backwards through the fibre walls inthe proximal end of the fibre bundle Filtrate is produced in one end ofthe shell and used to backwash the fibres at the other end of the shell.

A distinction is made between the term "reversed flow (filtering)" asused by Breslau and the description of reversing the direction of flowwhile no filtering is occurring as hereinafter described.

The prior art also contains a number of references to filter systemswhich utilize pressure variations arising,

For example, German specification No. 2,833,994 discloses a filtrationprocess in which two fluid streams flow countercurrent to each other oneither side of a filter medium. The flow of filtrate is subjected to aseries of reductions of the flow cross section. These reductions withthe associated acceleration in velocity induce a region of low pressurebelow the membrane, causing a flow of fluid through the membrane

Netherlands specification No. 7,604,657 discloses a method for cleaningtubular membranes in which gas is dissolved in a liquid under pressure.The liquid is fed past the membrane and the pressure is reduced so thatgas is released as small bubbles which lift solids from the membrane andcarry them away.

Similarly, the feeding of gas - liquid mixture to the surface of themembrane is taught by Japanese specifications 61-129094 and 56-024006.

The cleaning of dead-end fibres dangling in a pot by ga cleaning causingwrithing of the fibres is disclosed in British patent No. 2,120,952.Japanese specification 60-137404 teaches the installation of specialequipment to vibrate dead-end fibres hanging in a pot during backwashand Russian specification No. 715,105 discloses air pulsing of washwater applied to a granulated filter.

Japanese specification No. 53-042186 teaches the periodic reversal ofdirection of flow of feed liquid in a membrane plate separator. Japanesespecification No. 61-101209 discloses a method of applying a vacuum toeliminate air from the pores of a hydrophobic membrane.

Japanese specification No. 47-021748 discloses the reversal ofapplication of air pressure. First air pressure drives liquid throughthe membrane. When backwashing with filtrate is required, the airpressure is applied to the filtrate. When a flow meter indicatessufficient washing, the air pressure is again applied to the feed sideto restart the filtration.

The article "Anti-fouling Techniques in Cross-flow Microfiltration" byMilisic & Bersillon, Filtration &

Separation, November/December 1986, pp 347-349, teaches pulsing thefeedstream as it is applied during normal filtration.

Banks of fibres in shell filter cartridges are frequently arranged inparallel. When one shell develops a blockage, flow bypasses this fibrebundle, the velocity slows, and the blockage becomes self-increasingthrough the system.

The need to optimize the frequency of cleaning cycles to maximizefiltrate flow is discussed in International Patent Application No.PCT/AU84/00192.

For the procedure described to be successful, the fibres must beelastic. For practical considerations of each of manufacture andresistance to acid cleaning that must be applied eventually, and forstrength, the fibres are generally chosen to be a thermoplastic such apolypropylene. Such thermoplastics are fundamentally hydrophobic andmust be wetted before they can be used to filter aqueous feedstockstreams.

The application of backwashing gas as described above has the undesiredeffect of partially drying the fibres. Small bubbles of gas are retainedin the pores in the walls of the fibres where they effectively blockfiltration. The filtrate flux is initially high at the start offiltration, but rapidly drops as the fibres foul with solids. Theapplication of the two stage backwashing regime restores the filtrateflux to a high value that is however, not as high as the initial value.At each stage this slight diminution of flux reduces the filtrationcapacity of the fibres. Eventually chemical cleaning and/or rewetting isrequired which is expensive and time consuming.

International Patent Application No. PCT/US83/02004 discloses thepressurized initial wetting of fibres in relation to cartridge unitsthat are intended for a special use such as with blood, and which can beprewetted before shipment. However, in industrial situations, cartridgesmay be used for many applications that are not specified at the time ofmanufacture of the cartridge. For applications such as food use, thepresence of extraneous wetting agents such as surfactants must beavoided and there is a need to wet the fibres with the liquid to befiltered. In these cases it is impractical to wet the fibres duringmanufacture. They must be wetted in place, immediately prior to use. Theprocedure described in our International Patent Application No.PCT/AU86/00049 utilizes a flow of feedstock to wash away the dislodgedsolids. However, it is sometimes necessary that the solid material berecovered in a dryer state than is the case with the processes describedin our above International Patent Applications. This is particularlyuseful where solids recovery and dewatering are important.

According to the invention there is provided a method of operating afilter having elastic, porous, hollow fibres within a shell or housingcomprising the steps of:

(i) introducing a liquid suspension feedstock into the shell or housingand directing said feedstock to the outer surface of the fibres whereby:

(a) some of said feedstock passes through the walls of the fibres to bedrawn from the fibre lumens as a filtrate or permeate,

(b) some of the solids in said feedstock are retained on or in the poresof the fibres, with the non-retained solids being discharged from theshell or housing with the remainder of said feedstock,

(ii) periodically cleaning away the retained solids by:

(a) introducing a pressurized gas into the fibre lumens which passesthrough the walls of the fibres to dislodge the retained solids, and,

(b) varying the pressure within the shell whilst the gas is beingintroduced into the lumens.

According to another aspect of the invention, there is provided a methodof operating a filter having a plurality of elastic, microporous hollowfibres with a shell or housing comprising the steps of:

(i) introducing a liquid suspension feedstock into the shell or housingand applying said feedstock to the outer surface of the fibres whereby:

(a) some of said feedstock passes through the walls of the fibres to bedrawn from the fibre lumens as a permeate,

(b) some of the solids in said feedstock are retained on or in the poresof the fibres with the non-retained solids being removed from the shellwith the remainder of said feedstock,

(a) introducing a pressurized liquid through the fibre lumens whichpasses through the walls of the fibres to wash out at least some of theretained solids and then,

(b) introducing through the fibre lumens a pressurized gas which passesthrough the walls of the fibres and stretches elastically at least someof the pores to dislodge any solids retained in those pores and whichwashes the external walls of the fibres, the gas being applied at apressure which is sufficient to overcome the resistance to gas flow ofthe surface tension of the continuous phase of the filtrate within thepores of the membranes, and,

(c) varying the pressure within the shell whilst the pressurized gas isbeing introduced into the lumens.

The pressure within the shell may be varied during cleaning in a numberof ways such as by :increasing the pressure within the shell above thenormal gaseous cleaning pressure and then returning the pressure to thenormal gaseous cleaning pressure or by decreasing the pressure withinthe shell below the normal gaseous cleaning pressure and then returningthe pressure to the normal gaseous cleaning pressure.

The pressure within the shell may be increased by terminating theoutflow of feed and then returned to normal gaseous cleaning pressure byrecommencing flow of feed in either the same or the reverse direction.

The pressure within the shell may be decreased by terminating the inflowof feed and the return to normal gaseous cleaning pressure can beachieved by resuming inflow of feed in the same or the reversedirection.

The methods of the invention may be modified by terminating the inflowof feed before commencing the gaseous backwash step to effect a drybackwash. The feed flow may also be replaced by a high or low pressuregas through the inlet to the shell so as to assist the discharge of theretained solids.

All the above variations in the mode of operating the filter during thecleaning cycle may be repeated a number of times during gaseouscleaning.

In one form of the invention, the shell is pressurized by terminatingfeed flow before the pressure variation step and the pressure isreleased by recommencing feed outflow prior to the application of thepressure variation step.

In a modification of this form of the invention, the pressure isreleased at both the feed and recirculation ends of the shell.

The methods of the invention may also be modified by including a step ofpressurizing the fibres after the completion of the backwash and thenreleasing that pressure to remove trapped air from the pores of thefibres. The step of pressurizing the fibres may be carried out byterminating the feed inflow and feed outflow and the pressure may bereleased by recommencing feed inflow with or without recommencement offeed outflow. The pressurization of the fibres is carried out whilstlumen flow is blocked preferably in a pulsing fashion.

To carry out the pressurization, after the backwash cycle has beencompleted, the feedstock and filtrate flow are blocked. A hydraulicpressure preferably from a piston of pressurized gas is applied toeither the filtrate side of the fibres or the feedstock side of thefibres, or both. Thus pressure is applied to the fibres and thecompressible gas contained in the pores of the fibres is reduced involume or dissolved in the liquid in the fibres due to its greatersolubility under pressure. On resumption of feed flow the gas isexpelled In some circumstances it may be preferable to drain the fibrelumens before commencement of the gaseous backwash step. Furthermore, itmay be advantageous to drain the shell before commencement of backwash.

According to another aspect of the invention, the introduction of thepressurized gas for cleaning includes the steps of:

(a) initially applying the gas at a pressure below the bubble point ofthe walls of the fibres so as to displace any liquid from the fibrelumens,

(b) terminating feed inflow and outflow,

(c) increasing the pressure of the gas above the bubble point of thewalls of the fibres, and,

(d) recommencing feed inflow and outflow to allow the trapped gas toescape substantially uniformly through the fibre walls.

Preferably, the introduction of the pressurized gas during the drybackwash includes the steps of:

(a) introducing another gas into the shell side of the fibres at apressure substantially the same as the lumen cleaning gas,

(b) terminating the flow of the shell side gas, opening the shell inletand/or shell outlet to release the gas pressure on the shell side of thefibres and to allow the lumen gas to escape substantially uniformlythrough the fibre walls.

The filter may be operated in a cross flow mode or in a dead-endfiltering mode with no outflow of feed and solids from the shell, duringthe dead-end filtration mode.

In yet another embodiment of the invention, the backwashing cleaningstep is enhanced by discharging through both the shell inlet and outletand feeding through an additional line connected to the shell betweenthe shell inlet and outlet.

The invention also includes apparatus for carrying out the methodsdescribed above.

In order that the invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings in which:

FIG. 1 is a schematic view of a hollow fibre crossflow filter shown inits operating mode,

FIG. 2 is a schematic view similar to FIG. 1 with the filter shown inits gas backwash cleaning mode,

FIG. 3 is a graph of clarified liquid flux against time for a hollowfibre cross-flow concentrator,

FIG. 4 is a partly broken away view of one end of the filter cartridgeshown in FIGS. 1 and 2,

FIG. 5 is a view similar to FIG. 4 of a modified form of the cartridgeend,

FIG. 6 is a view similar to FIG. 4 of a further modified form of thecartridge end,

FIG. 7 is a schematic diagram of a filtering installation for theapplication of the method of the invention,

FIG. 8 is a simplified schematic diagram of a modified form of theinstallation shown in FIG. 7,

FIG. 9 is a graph of filtrate flux against time for a filtration systemfor three modes of operation,

FIG. 10 is a graph of filtrate flux against time for a filtration systemcomparing two modes of operation,

FIG. 11 is a graph of flux against time for the filtration of afeedstock using mode 1(b) backwash,

FIG. 12 is a graph of flux against time similar to FIG. 11 but showing amode 2(b) backwash, and,

FIG. 13 is a graph similar to FIG. 12 but with four cycles of reverseflow of feedstock followed by a backwash of mode 2(b).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hollow fibre cross-flow concentrator 10 shown in FIGS. 1 and 2includes a cartridge shell 11 within which is positioned a bundle ofhollow, porous, polymeric fibres 12. In this instance, each fibre ismade of polypropylene, has an average pore size of 0.2microns, a wallthickness of 200 microns and a lumen diameter of 200 microns. There are3,000 hollow fibres in the bundle 12 but this number as well as theindividual fibre dimensions may be varied according to operationalrequirements.

Polyurethane potting compound 13, 14 holds the ends of the fibres 12 inplace without blocking their lumens and closes off each end of the shell11. The liquid feed suspension to be concentrated is pumped into theshell 11 through feed suspension inlet 15 and passes over the externalwalls of the hollow fibres 12. Some of the feed suspension passesthrough the walls of the fibres 12 into the lumens of the fibres to bedrawn off through the lumen outlet posts 16 and 18 as clarified liquid.

The remaining feed suspension and some of the rejected species flowsbetween the fibres 12 and leaves the shell 11 through outlet 17. Theremainder of the rejected species is held onto or within the fibres oris otherwise retained within the shell.

In order to remove the retained species, lumen outlet port 16 is closedso that the flow of clarified liquid is stopped. Pressurized clarifiedliquid is then introduced into the lumens through lumen inlet port 18 tostretch substantially all of the pores and to wash them with at leastthe total pore volume of clarified liquid. Upon completion of theclarified liquid purge, compressed gas is introduced through lumen inletport 18, along the lumens of the fibres 12 and through the walls of thefibres into the feed suspension/concentrated steam causing violentbubbling which purges the shell of any retained species which may havebuilt up on the outer walls of the fibres or may have been washed fromwithin the pores of the fibres by the clarified liquid purge.

In one embodiment of the invention (which is particularly suitable forlong thin fibres), the compressed gas is introduced through inlet 18 andalong the lumens of the fibres 12 after opening the lumen outlet port 16for a period sufficient for all the liquid to be removed from the lumensthrough no gas penetrates the pores of the fibres at this stage. Theport 16 is then closed and the liquid-filled shell is sealed by closingshell inlet 15 and shell outlet 17. Gas still cannot penetrate theporous walls even though the gas pressure is now raised well above thenormal bubble point of the fibre walls because the liquid within theshell is relatively incompressible. A reservoir of high pressure gas isthus accumulated in the fibre lumens.

The shell outlet 17 is then opened which allows gas to penetrate thepores along the whole length of each fibre. Initially, the surge ofbubbling gas is substantially uniform but ultimately is slower at theend remote from lumen inlet port 18 due to the viscous pressure dropalong the thin fibres. In extreme cases, it is desirable to admit gasthrough both lumen ports 16 and 18 after carrying out the abovedescribed pressurized, trapped gas operation.

It is preferred that the resumption of feed suspension flow aftergaseous cleaning be delayed for sufficient time to enable the pores thathave been stretched by the gas to recover to their original size so thatover-sized particles from the feed suspension will not be able to passinto or through the enlarged pores.

FIG. 3 shows the effect of the solid discharges described in relation toFIG. 2 upon the rate of production of clarified liquid. Curve A showsthe decay of clarified liquid flux against time without discharge ofsolids, whereas Curve C show the recovery of clarified liquid flux aftereach combined liquid and gaseous discharge cycle. Although the dischargeof solids returns the clarified liquid flux to almost the initial value,a decrease in efficiency may occur over an extended period of timenotwithstanding successive discharges. The slight reduction in thefiltration capacity of the fibres at each stage eventually results in aneed for chemical cleaning, which is expensive and time consuming.

One end of the filter cartridge shown in FIGS. 1 and 2 is shown on anenlarged scale in FIG. 4. It will be seen that the tubular shell 11projects into a housing 20 that carries the feed suspension outlet 17and filtrate discharge port 16. The housing 20 is made of two parts 21,22 within which is located a collar 23 that supports a spigot 24 leadingto the outlet 17. In this embodiment of the cartridge, the inner end 25of the spigot 24 is flush with the inner surface of the collar 23 andthe shell 11 projects into housing part 21 with its end 26 terminatingbeyond the spigot 24.

The modified version of the end of the cartridge shown in FIG. 5 issubstantially similar to that shown in FIG. 4, the differences beingthat the inner end 26 of the shell 11 does not project into the housingpart 21 and that the inner end 25 of the spigot 24 projects beyond theinner surface of the collar 23 and that the inner end 25 of the spigothas a cut-away portion 27.

The modified version of the end of the cartridge shown in FIG. 6 issubstantially similar to that shown in FIG. 5, the difference being thatthe inner end 26 of the shell 11 does project into housing port 21 butterminates short of the spigot 24.

The techniques of the invention can be implemented using theinstallation shown in FIG. 7. In FIG. 7, feed line 50 from the tank 51to feed pump 52 and check valve 53 branches into lines 54 and 55. Manualvalve 56 in line 54 is closed during normal filtration. Feed in line 55passes through feed valve 63 and into the shell side of filter 57through feed line 64. Feed discharged from the filter 57 flows throughline 58 into line 59 having a pressure gauge 60 and then into the mainreturn line 61 which has a manual control valve 62.

Filtrate from the filter 57 is discharged through filtrate lines 65 and66. Filtrate from line 65 passes through line 67 which has a manualcontrol valve 68 and line 84 which has a pressure gauge 69 to filtratedischarge line 70 which also has a manual control valve 71. Filtratefrom line 66 is also discharged through line 70.

Gas may be introduced into line 84 from line 72 which contains a checkvalve 73. A discharge line 74 is connected to the feed line 64 andcontains a manual drain valve 75 and pressure gauge 76. The dischargeline 74 is connected to the main discharge line 77 as is line 78 whichhas a manual drain valve 79. A return line 80 connected between thefiltrate lines 65 and 67 and tank 51 has a manual valve 81. Anadditional gas line 82 controlled by valve 83 enters feed line 50downstream of the check valve 53.

During normal filtration pump 52 is on and valves 63, 71, 68 and 62 areall open and valves 56, 79, 75 and 81 are closed. Desired operatingpressures are set by adjusting manual valves 63 and 62.

The filter installation shown in FIG. 7 can operate in a number ofdifferent modes of backwashing by manipulating the various valves,altering the flow pattern and by changing the identity (liquid or gas)of the medium in one, some or all of the lines.

In brief terms, these modes of backwash may be identified as:

    ______________________________________                                        MODE 1 NORMAL BACKWASH                                                        MODE 2 PRESSURE INCREASE WITH REVERSE FLOW                                           OF FEED DURING BACKWASH                                                MODE 3 PRESSURE DECREASE WITH PULSING FEED                                           INFLOW                                                                 MODE 4 PRESSURE INCREASE WITH PULSING FEED                                           OUTFLOW                                                                MODE 5 PRESSURE DECREASE WITH REVERSE FLOW                                           OF FEED                                                                MODE 6 RELEASING SHELL PRESSURE AT BOTH                                              INLET AND OUTLET POINTS DURING                                                BACKWASH                                                               MODE 7 NO LIQUID FEED FLOW DURING BACKWASH                                           (DRY BACKWASH)                                                         MODE 8 REWETTING PRESSURISATION                                               ______________________________________                                    

All the first seven modes may be effected by either first draining thelumens or not draining the lumens. Thus, the above modes will beidentified as (a) when the lumens are drained and (b) where the lumensare not drained where such distinction is appropriate.

It is convenient to describe Mode 2 before Mode 1 as the latter consistsof seven stages which are common to the ten stages of Mode 2.

MODE 2 - PRESSURE INCREASE WITH REVERSE FLOW OF FEED

The reverse flow of feed during backwash mode consists of ten stages.During stage 1, pump 52 is off, valves 63, 56, 79, 75, 71, 68 and 62 areclosed and valve 81 is open. Low pressure gas is introduced via line 72and check valve 73. The gas flows through lines 84 and- 66 and into thebottom filtrate port of cartridge bank 57. Filtrate from within thelumens is flushed out and exits via lines 65 and 80 back to tank 51.During this stage, the gas pressure is held low, below the bubble point,so that there is no gas breakthrough across the membrane.

During stage 2, pump 52 remains off, valve 73 remains open and valves63, 56, 79, 75, 71 and 62 remain closed. Valve 81 is closed and valve 68is opened. High pressure gas is then introduced via lines 72, 67, 84, 66and 65. This pressurizes both the lumen side and the shell side of thecartridge bank 57, typically to 600KPa(g).

During stage 3, pump 52 remains off, valves 63, 56, 75, 71, 81 and 62remain closed and valves 68 and 73 remain open. Valve 79 is opened torelease the shell side pressure with high pressure gas still beingapplied to the lumens via lines 72, 67, 84, 66 and 65. The gas passesthrough the pores of the fibres to the shell side of the filter 57 andexits via lines 58, 59, 78 and 77. The purpose of this third stage is todislodge accumulated solids from the outside of the fibres.

During stage 4, the valve settings are the same as for Stage 3 exceptfor valve 63 which is now opened. Pump 52 is turned on and remains onuntil the next backwash sequence is started. High pressure gas is stillapplied to the lumens via lines 72, 67, 84, 66 and 65. The purpose ofthis fourth stage is to wash dislodged solids to drain, via lines 58,59, 78 and 77.

During stage 5, valves 68 and 73 remain open and valves 56, 71, 75, 81and 62 remain closed. Valves 63 and 79 are closed and high pressure gasis still applied to the lumens via lines 72, 67, 84, 65 and 66 whichpressurizes both the lumens and the shell side of the cartridge bank 57.

During stage 67, the valve settings are the same as for Stage 5 exceptfor valves 56 and 75 which are opened to release the shell-side pressurewith high pressure gas still applied to the lumens. The flow of feeddown the cartridge bank 57 is now reversed, and the dislodged solids arecarried away via lines 64, 74 and 77.

The seventh stage is the same as the fifth stage and the eighth stage isthe same as the fourth stage. The total sequence of stages 4, 5, 6, 5, 4is repeated one or more times.

Stage 9 is the same as stage 4 except the high pressure gas is turnedoff to remove residual gas in the feed stream to drain via lines 58, 59,78 and 77.

For stage 10, valves 63, 68 and 81 are open, valves 56, 79, 75, 71 and62 are closed and the high pressure gas remains off to remove residualgas in filtrate lines 65, 66, 67, 84 and 80. At the completion of stage10, the installation is returned to normal filtration.

MODE 1 - NORMAL BACKWASH

The normal backwash mode consists of stages 1 to 4 and 9 to 10 of Mode2. Thus, during stage 1, low pressure gas is introduced into the lumensto drain filtrate from the lumens. During stage 2, the gas pressure isincreased to pressurize both the lumen side and the shell side of thefilter 57.

At stage 3, drain valve 79 is opened to release the shell side pressurewhilst high pressure gas is still being applied to the lumens todislodge accumulated solids from the outside of the fibres. Feed valve63 is opened and the pump 52 turned on in stage 4 to wash the dislodgedsolids through drain valve 79 to discharge line 77.

The high pressure gas is then turned off (stage 9) and residual gas inthe feed stream is discharged through lines 59 and 78 to discharge line77. In the final stage (stage 10), residual gas in the filtrate lines65, 66 and 67 is discharged through line 80 to the tank 51. At thecompletion of stage 10, the installation is returned to normalfiltration.

MODE 3 - PRESSURE DECREASE WITH PULSING FEED INFLOW

In this mode, stages 1 to 4 are the same as those described above inrelation to Mode 2. Thus, low pressure gas is used to drain the lumens(stage 1), high pressure gas is used to pressurize both the lumen sideand the shell side of the filter (stage 2), drain valve 79 is opened torelease the shell side pressure to dislodge accumulated solids (stage 3)and feed flow recommenced through feed valve 63 to wash the solidsthrough the drain valve 79 to discharge line 77 (stage 4.).

Stage 5 is the same as stage 4 except that the feed valve 63 is closedto drop the shell side pressure of the cartridge 57 below the normalgaseous cleaning pressure.

Stage 6 of this mode is the same as stage 4, that is, valve 63 is openedso that the pressure on the shell side returns to the normal gaseouscleaning pressure. Stage 7 of this mode is the same as stage 5 and stage8 of this mode is the same as stage 4.

The total sequence of stages 4, 5, 4 in order (i.e. stages 4 to 8) isrepeated one or more times. Stages 9 and 10 of this mode are the same asstage 9 and 10 of Mode 2.

MODE 4 - PRESSURE INCREASE WITH PULLING FEED OUTFLOW

In this mode, stages 1 to 4 are the same as Stages 1 to 4 of Mode 2.

Stage 5 of this mode is the same as stage 4 of this mode except thatdrain valve 79 is closed so that the pressure on the shell side of thefilter cartridge 57 is increased from the normal operating gaseouscleaning to the pressure on the lumen side.

Stage 6 of this mode is the same as stage 4 of this mode. Thus, valve 79is opened and feed flow recommenced. The pressure on the shell side ofthe cartridge drops back to the normal gaseous cleaning pressure.

Stage 7 of this mode is the same as stage 5 of this mode and stage 8 ofthis mode is the same as stage 4 of this mode.

The total sequence of stages 4, 5, 4 in order (i.e. stages 4 to 8) isrepeated one or more times with the action to initiate the pressurevariation cycle always being applied at the same end of the shell.Stages 9 and 10 are the same as stages 9 and 10 of Mode 2.

MODE 5 - PRESSURE DECREASE WITH REVERSE FLOW OF FEED

This mode follows stages 1 to 5 of Mode 3. In stage 6, valves 56, 75 and68 are open and valves 63, 79, 73, 81 and 62 are closed. Pump 52 is onand high pressure gas is still applied through line 72. Dislodged solidsare removed through lines 64, 74 and 77.

Stage 7 of this mode is the same as stage 5 of Mode 3 and stage 8 ofthis mode is the same as stage 4 of Mode 3.

The total sequence of stages 4, 5, 6, 5, 4 in order (i.e. stages 4 to 8)is repeated one or more times.

It should be noted that the pressure cycle introduced after stage 4 ofModes 2 to 5 can be either pressure increases from normal gaseouscleaning pressure and, then returns to the normal gaseous cleaningpressure (Modes 2 and 4), or, pressure decreases from normal gaseouscleaning pressure, and then returns to normal gaseous cleaning pressureModes 3 and 5).

With Modes 2 and 4, after the pressure increase, the pressure release isalways at the feed outflow end of the shell. With Mode 4, the feedoutflow is always at the same end of the shell with Mode 2, the feedoutflow alternates from one end of the shell to the other on each cycle.

With Modes 3 and 5, the pressure cycle is always initiated at the feedinflow end of the shell. With Mode 3 the feed inflow is always at thesame end of the shell. With Mode 5 the feed inflow alternates from oneend of the shell to the other on each cycle.

MODE 6 - RELEASING SHELL PRESSURE AT BOTH INLET AND OUTLET POINTS DURINGBACKWASH

This mode can be applied to Modes 1 to 5. The pressure release refers tothe release of the pressure built up within the shell during stage 2 ofModes 1 to 5 which is different from the pressure variation cycleintroduced after stage 4. To achieve the release of pressure at both theinlet and outlet points of the shell stage 3 is modified by additionallyopening valve 75 to equalize the trans membrane pressure down the filtercartridge bank 57.

MODE 7 - NO LIQUID FEED FLOW DURING BACKWASH(DRY BACKWASH)

In this mode, the feed pump 52 is off for the entire backwash cycle toeffect a dry blow back mode and dry blow back gas dislodges accumulatedsolids and carries these solids away to drain.

The dry blow back mode can be applied using the stages of Mode 2, Mode3, Mode 4 and Mode 5 with or without Mode 6 by replacing flow of liquidfeed with a flow of high pressure gas through line 82 and check valve 83and line 50 in all relevant stages of Mode 2, Mode 3, Mode 4 and Mode 5with or without Mode 6.

A modification of Mode 7 can be made in Stages 1 and 2. In thismodification gas at approximately the same pressure as that applied tothe lumens is simultaneously applied to the shell as a means ofpressurizing both the lumen side and the shell side of the cartridgebank. This can be achieved by opening valves 63 and 56 and applying gasthrough line 82 and check valve 83. In stage 3, valve settings return tothose of stage 3, Mode 2, and the gas to the shell is discontinued.

In a further modification cf Mode 7, which can, be applied to Mode 2 andMode 4 with or without Mode 6, there is no feed flow and no secondarygas flow through line 82.

Thus, the dry blow back mode 7 can be effected in a number of sub modesas follows:

    ______________________________________                                        Sub Mode     Parent Mode                                                                              Lumen Drained                                         ______________________________________                                        7(a)         1a         Yes (a)                                               7(b)         1b         No (b)                                                7(c)         2a         Yes (a)                                               7(d)         2b         No (b)                                                7(e)         3a         Yes (a)                                               7(f)         3b         No (b)                                                7(g)         4a         Yes (a)                                               7(h)         4b         No (b)                                                7(i)         5a         Yes (a)                                               7(j)         5b         No (b)                                                ______________________________________                                    

In addition sub Modes 7(a) to 7(j) may be carried out with the shelldrained (sd) or with the shell not drained (snd).

MODE 8- REWETTING PRESSURIZATION

A rewetting of the membranes stage can follow any of the above modes.The rewetting stage may be applied when a backwash cycle is complete, orat any other time. The rewetting cycle consists of subjecting the fibresto a pressurization followed by a fast release of pressure to removetrapped air which is blocking the fibres. This can be achieved in thefollowing three steps:

In step 1, valves 79, 75, 71, 81 and 62 are closed, valves 63, 56 and 68are open and high pressure gas is introduced to the filtrate side ofcartridge 57 via lines 72, 67, 84, 65 and 66. This pressurizes both thelumen side and the shell side of cartridge 57.

In step 2, the flow of high pressure gas is stopped and all valvesettings are the same as for step 1 except that valve 81 is open. Thisreleases the pressure inside cartridge 57, removing trapped bubbles ofgas from within the fibres. Step 3 is a return to normal filtration.

A modification of the rewetting Mode 8 is to introduce the high pressureair on the shell side instead of the filtrate side of cartridge 57. Thiscan be achieved in the following three steps:

In step 1, valves 79, 75, 71, 81 and 62 are closed, valves 63, 56 and 68are open and high pressure gas is introduced to the shell side ofcartridge 57 via lines 82, 50, 54, 55, 64 and 58 to pressurize both theshell and the lumen sides of cartridge 57.

In step 2 the flow of high pressure gas is stopped. All valve settingsare the same as for Step 1 except that valve 79 is open to release thepressure inside cartridge 57 and removes trapped bubbles of gas fromwithin the fibres. Step 3 is a return to normal filtration. Therewetting pressurization cycle may be performed one or more times.

The combination of the backwash and reversal of the direction of flowproduces an effect that is greater than that expected by the addition orsupposition of the results of backwashing on the results of reversingthe direction of the flow of feed. The result is somewhat unexpected butis possible because the application of the technique, as has beenobserved in transparent shell cartridges, establishes new flow patterns,thereby reducing the self-increasing effect of blocked cartridges thatwas discussed earlier The increased turbulence created by thesimultaneous application of the two techniques clears blockages from thecartridge instead of allowing material to build up on previouslydeposited material.

During the filtering stage where flow is in one direction, there is asmall pressure drop in feedstock pressure along the cartridge. Thisdifference in pressure between distal and proximal ends of the cartridgeincreases during the application of the gaseous backwash. Thus the gasat the distal end of the cartridge faces a lower transmembrane pressuredrop than the gas at the proximal end of the cartridge, and more bubblespass through the fibre walls at the distal end of the cartridge. Thereversal of the direction of the flow of feed applied during gaseousbackwash reverses the pressure difference effect and allows a more evendistribution of bubbles passing through the walls of the fibres.

The relative effectiveness of the liquid and gaseous backwashes and ofreversal of the direction of flow of the feedstock depends on the natureof the suspension being filtered. Caking deposits are better removed bygaseous backwash combined with a reversal of direction of flow offeedstock. Indeed it is for such deposits that form clots that thetechnique is particularly successful when compared with other methods.Backwash alone loosens the retained solids which are then quicklyredeposited on the fibres as soon as filtration is recommenced. Theapplication of reversal of direction of flow of feed creates turbulencealong the outer walls of the fibres and carries away the clotted solidmaterial.

FIG. 8 shows a modified installation substantially similar to that ofFIG. 7 and, as such, most components have been omitted. Feed to theshell 40 is applied through line 41 and a three-way valve 42 to feedinlet 43. Feed is discharged from the shell through feed outlet line 44having a valve 45. Valve 48 is closed during the filtering operation.

A third port is connected to line 46 leading from the feed inlet valve42. Discharge line 47 having a valve 48, is connected between the feedinlet 43 and feed outlet line 44 downstream of the discharge valve 45.

With such a filter, the method described in respect of the FIG. 7installation is modified in that after the release of the pressure onthe outer surface of the fibres (i.e. after stage 3 of Mode 2), the feedfrom the pump is directed to the third port through line 46 so as toflush out both ends of the shell or housing through discharge line 47 inthe case of the inlet end of the shell and through the normal feedoutlet line 44 at the other end of the shell.

In order that the invention may be more readily understood, referencewill now be made to the following examples which were carried out usinginstallations having sufficient of the features of the installation ofFIG. 7 to enable the particular Mode to be effected.

In all cases, the filter cartridge contained a bundle of about 3,000polypropylene hollow fibres with feed being applied in a cross-flowfashion to the outer surface of the fibres and filtrate being withdrawnfrom each end of the fibre lumens. The cartridge end design of FIG. 4was used in Examples 1 to 6 and 9, that of FIG. 5 was used in Examples 7and 8 and that of FIG. 6 was used in Examples 10 and 11.

EXAMPLE I: MODES 1(b), 2(b) and 4(b)

This experiment was conducted to compare the effectiveness of the normalMode 1(b), the reverse flow Mode 2(b), and the pulsed Mode 4(b).

A suspension containing ferric hydroxide was made by mixing 360 ml"Ferriclear" and 1080 g sodium hydrogen carbonated in 20 litres water,to precipitate 199.8 g ferric hydroxide.

In the normal mode (1b), the feed inlet pressure was 50 KPa and the feedoutlet pressure zero. Air at 600 KPa was blown back for 6 seconds. Thetotal time of the backwash and air removal was 40 seconds. The cycle ofMode 1(b) followed by mode 8 was repeated every 10 minutes.

Backwash Modes 2(b) and 4(b) were carried out.

After each of the above modes, the filter was subjected to rewettingmode 8.

The three modes may be compared by comparing the filtrate flow ratesafter 10 minutes, and recovery after air blowback for severalconsecutive cycles. The results are set forth in Table I where:

1(b) is the normal mode, with lumens not drained 2(b) is the pressureincrease with reverse flow of feed mode, with lumens not drained

4(b) is the pressure increase with pulsing feed outflow mode, withlumens not drained

It can be seen from the following Table I that Mode 2(b) recovered thefiltrate flow rate (flux) to a higher value than Mode 4(b) and that thiswas in turn more effective than Mode 1(b). The results are shown in FIG.9.

                  TABLE I                                                         ______________________________________                                        Time                 Backwash Mode applied                                    Mins       Flux 1/hr after 10 min                                             ______________________________________                                         0         740                                                                10         295       1(b)                                                      0         760                                                                10         300       1(b)                                                      0         750                                                                10         330       1(b)                                                      0         760                                                                10         360       1(b)                                                      0         760                                                                10         360       2(b)                                                      0         900                                                                10         335       2(b)                                                      0         920                                                                10         340       2(b)                                                      0         950                                                                10         300       2(b)                                                      0         950                                                                10         340       4(b)                                                      0         820                                                                10         320       4(b)                                                      0         815                                                                10         345       4(b)                                                      0         810                                                                10         360       4(b)                                                      0         830                                                                10         360       Nil                                                      ______________________________________                                    

EXAMPLE 2 MODES 1(b), 4(b) and 2(b)

A mixture of 50 g diatomite ("Whitco") and 50 g bentonite in 201 waterwas filtered repeatedly to test the effectiveness of the modes ofbackwashing. All feed, filtrate and backwashed material was returned tothe feed tank. The temperature of the feed was kept constant in therange of 25.5° C. to 26° C. by a copper coil heat exchanger using coldtap water. The modes tested were the normal backwash, Mode 1(b), thepulsed mode 4(b) and the reverse flow Mode 2(b). Each backwash wasfollowed by rewetting sequence mode.

The results in which the time taken to apply the backwash has beendeleted from the time figures are shown in Table II where:

1(b) is the normal mode, with lumens not drained

4(b) is the pressure increase with pulsing feed outflow mode, withlumens not drained

2(b) is the pressure increase with reverse flow of feed mode, withlumens not drained

                  TABLE II                                                        ______________________________________                                        Time           Flux    Backwash                                               (mins)         (1/hr)  Mode                                                   ______________________________________                                         0             850                                                            10             420     2(b)                                                   10             840                                                            20             420     2(b)                                                   20             830                                                            30             400     2(b)                                                   30             830                                                            40             420     Nil                                                     0             850                                                            10             425     1(b)                                                   10             800                                                            20             405     1(b)                                                   20             780                                                            30             410     1(b)                                                   30             750                                                            40             405     Nil                                                     0             850                                                            10             400     4(b)                                                   10             820                                                            20             400     4(b)                                                   20             800                                                            30             395     4(b)                                                   30             800                                                            40             395     Nil                                                    ______________________________________                                    

EXAMPLE 3 MODES 4(b) and 2(b)

A feedstock consisting of muddy water with a turbidity of 420 NTU wasused in carrying separate examples in respect of Mode 4(b) and Mode 2(b)each of which was followed by Mode 8. All filtrate and solid materialblown off by the backwash was returned to the feed tank. The filtrate atall times was optically clear.

The filtration time between backwashes was 7 minutes. Each backwash tooka total of about one and a half minutes to apply, however air onlypassed through the membrane for ten seconds.

The results are given in the following Tables III and IV and are graphedin FIG. 10.

                  TABLE III                                                       ______________________________________                                        PRESSURE INCREASE WITH PULSING FEED                                           OUTFLOW MODE (4b)                                                             Time (mins) Filtrate Flow Rate (Liters/hour)                                  ______________________________________                                        0.          1100                                                              7.0         600                                                               Backwash                                                                      8.75        800                                                               16.0        500                                                               Backwash                                                                      17.45       650                                                               23.25       500                                                               Backwash                                                                      25.5        600                                                               32.0        450                                                               Backwash                                                                      33.75       600                                                               40.5        450                                                               Backwash                                                                      42.5        550                                                               50.0        450                                                               Backwash                                                                      52.5        550                                                               ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        PRESSURE INCREASE WITH REVERSE FLOW OF                                        FEED (Mode 2(b))                                                                                                Filtrate Flow                                        Filtrate Flow Rate       Rate                                        Time (mins)                                                                            (Liters/hour) Time (mins)                                                                              (Liters/hour)                               ______________________________________                                        60.0     400           Backwash                                               Backwash               146.5      800                                         61.5     600           154.0      550                                         70.0     450           Backwash                                               Backwash               155.25     850                                         71.75    700           162.0      575                                         78.5     500           Backwash                                               Backwash               163.1      850                                         79.8     825           170.0      575                                         87.0     525           Backwash                                               Backwash               171.25     850                                         88.25    800           178.0      575                                         95.0     525           Backwash                                               Backwash               179.0      850                                         96.25    800           186.0      575                                         103.0    550           Backwash                                               Backwash               187 25     850                                         104.75   750           194.0      575                                         112.0    500           Backwash                                               Backwash               195.1      850                                         113.75   750           202.0      575                                         121.0    550           Backwash                                               Backwash               203.0      875                                         122.25   800           210.0      575                                         129.0    525           Backwash                                               Backwash               211.0      875                                         130.5    800                                                                  137.0    525                                                                  Backwash                                                                      138.25   850                                                                  145.0    550                                                                  ______________________________________                                    

EXAMPLE 4 MODE 7(b) (snd)

A suspension was made by mixing 50.1 g diatomaceous earth with 20 litresof water and then filtered.

After 10 minutes of cycling, the feed was blocked off at both the feedinlet to and the feed outlet from the cartridge. A backwash of Mode7(b), shell not drained, was performed. The backwash was collectedthrough the external drain line and filtered. The amount of drydiatomite recovered was 9.2 g. The recovery of diatomite was therefore18.4%. 1.8 litres of backwash were collected. The dryness of thecollected material was therefore 5.11 g/l.

EXAMPLE 5

Three batches of suspension were each made by mixing 48.7 g diatomitewith 20 litres of water. The temperature of the feed in the tank wasmaintained at 25° C. plus or minus 0.2 of a degree.

EXAMPLE 5 - Batch 1 MODE 1(b)

The initial water flux of the cartridge was 650 l/h. The trans membranepressure was 115 KPa, where the inlet pressure was 200 KPa, the feedoutlet pressure was 100 KPa, and the filtrate pressure was 35 KPa. After10 minutes of recycling the suspension through the filter cartridge asin Example 5, the liquid remaining in the feed tank was quite clear andit was concluded that nearly all the diatomite had been deposited on theoutside of the fibres.

Air was blown back through the membrane for about 15 seconds in backwashMode 1(b). 3.3 litres of backwash was collected yielding 18.8 g ofdiatomite. The recovery was 38.6%.

Three backwashes were performed with reversal of the direction of theflow of feed during the backwash to remove almost all of the remainingdiatomite.

EXAMPLE 5 - Batch 2 MODE 7(d) (snd)

The initial water flux of the cartridge was 680 l/h, the inlet pressurewas 200 KPa, the feed outlet pressure was 100 KPa, and the filtratepressure was 35 KPa. After 10 minutes of filtration of the batch, abackwash of Mode 7(d), shell not drained, was applied Air was blown backfor 20 seconds.

2.2 litres of backwash were collected yielding 14.4 g diatomite. Therecovery was 29.6%.

Again three reverse flow backwashes were performed to remove remainingdiatomite.

EXAMPLE 5 - Batch 3 MODE 7(g) (sd)

This batch was treated as for batch 2 except that before backwashing allfeed and filtrate lines were drained carefully so as to avoid disturbingthe diatomite on the surface of the fibres. Air was blown back for 1minute in backwash Mode 7(g). 0.27 litres of backwash were collectedyielding 12.5 g diatomite. The recovery was 25.7%.

It can be seem by comparing the dryness of the recovered solidsexpressed in g/l that Mode 7(g) (sd) gave a much drier material than themodes where the shell was not drained.

    ______________________________________                                        BATCH        MODE     RECOVERY g/l                                            ______________________________________                                        1            1b       5.7                                                     2            7d snd   6.5                                                     3            7g sd    46.3                                                    ______________________________________                                    

EXAMPLE 6

The investigation of Example 5 was repeated with ferric hydroxidesolution of pH7 containing 77.7 g ferric hydroxide in 20 litres ofwater.

The initial water flux of the cartridge was 820 l/h. The feed inletpressure was 200 KPa, the feed outlet was 100 KPa and the filtratepressure was 45 KPa The temperature of the feed in the tank wasmaintained at 25° C. plus or minus half a degree.

EXAMPLE 6 - Batch 1 MODE 2(b)

A 20 second backwash was performed in Mode 2(b). 6.26 litres of backwashwere collected with a ferric hydroxide recovery of 44.2%.

EXAMPLE 6 -Batch 2 MODE 7(d) (snd)

The initial water flux was 830 l/hr with temperatures and pressures asbefore. Air was blown back for 30 seconds in backwash 7(d), shell notdrained 1.8 litres of backwash were collected with a ferric hydroxiderecovery of 10.9%.

EXAMPLE 6 - Batch 3 MODE 7(g) (snd)

The initial water flux was 830 l/h with temperatures and pressures asbefore. With the pump turned off and the feed, feed return and filtratevalves closed, the filtrate lines and lumens were carefully drained. Airwas blown back for 30 seconds in backwash mode 7(g), shell not drained.1.8 litres were collected with a ferric hydroxide recovery of 13.3%.

EXAMPLE 6 - Batch 4 MODE 7(c) (sd)

The initial water flux was 820 l/h with temperatures and pressures asbefore. With the pump off, feed, feed return and filtrate, valvesclosed, and all lines, lumens and shell side of cartridge were carefullycleared of liquid, air was blown back for about one minute in backwashMode 7(c), shell drained. 230 ml backwash were collected yielding 7.44 gferric hydroxide, a recovery of 9.6%.

It may be concluded from these examples that application of mode 7results in the solids being recovered in a much more concentrated statebut sometimes at the expense of percentage recovery.

It can be seen by comparing the dryness of the recovered solidsexpressed in g/l that Mode 7(c) (sd) gave a much drier material than themodes where the shell was not drained.

    ______________________________________                                        BATCH        MODE      RECOVERY g/l                                           ______________________________________                                        1            2(b)      5.5                                                    2            7(b) (snd)                                                                              4.7                                                    3            7(g) (snd)                                                                              5.7                                                    4            7(c) (sd) 32.3                                                   ______________________________________                                    

EXAMPLE 7 MODE 1(a) FOLLOWED BY MODE 8

Tap water was filtered through three separate tube-in-shell filtercartridges each containing about 1m² of polypropylene porous hollowfibres prewetted with alcohol. The conditions of filtration were such asto maintain a trans membrane pressure of 100 KPa. The filtrate flowrate, or flux, was measured before an air backwash of mode 1(a) wasapplied, and again performing a mode 8 rewetting sequence. The fluxesrecorded for each cartridge are shown in the Table V.

                  TABLE V                                                         ______________________________________                                                                           FLUX                                                             FLUX         AFTER                                                FLUX        AFTER        PRESSUR-                                             BEFORE AIR  BACKWASH     ISATION                                    CARTRIDGE BACKWASH    MODE 1(a)    MODE 8                                     ______________________________________                                        1         10001/hr    2301/hr      11501/hr                                   2         10751/hr    3201/hr      12001/hr                                   3          7301/hr    1501/hr       8201/hr                                   ______________________________________                                    

The increased flow after pressurization compared with the flow beforebackwash is a result of the removal by backwash of fouling substancesthat had accumulated on the surface of the membrane and of air blockingthe membrane

EXAMPLE 8 MODE 8

Two new, dry cartridges, similar to those used in Example 7 wereseparately treated with pressurized water at 600 KPa for 2 to 3 secondsand the filtrate flow rate before and after pressurization is shown inthe following Table VI.

                  TABLE VI                                                        ______________________________________                                                   FLUX           FLUX                                                CARTRIDGE  BEFORE MODE 8  AFTER MODE 8                                        ______________________________________                                        1          01/hr          12501/hr                                            2          01/hr          13251/hr                                            ______________________________________                                    

EXAMPLE 9 MODES 1(a) and 2(a)

A one square metre MEMTEC cross-flow cartridge filter was run indead-end mode, i.e. no recirculation. The feed stream was mains tapwater having a typical turbidity of 6 NTU pH of 7.5 to 8. All tests wereperformed at approximately 200° C. With the feed recirculation valveclosed, a pressure regulating valve placed on the feed inlet wasadjusted to give a shell-side pressure of 250 KPa(g). Two backwasheswere tested with the intervals-between backwashes approximately sixhours. The backwashes tested were:

1(a) Normal backwash mode

2(a) Pressure increase with reverse flow of feed mode

The results are set out in TABLE VII.

                  TABLE VII                                                       ______________________________________                                        Case (i) Normal Backwash - Mode 1(a)                                          ______________________________________                                        Peak flux re-                                                                          880    720    660  620  600  580  530  530                           covery after                                                                  b/w(1/hr.m.sup.2)                                                             Flux after `t`                                                                         600    590    540  520  520  480  460  470                           hours of fil-                                                                 tration                                                                       (1/hr.m.sup.2)                                                                Interval be-                                                                            6      5      6    6    6    6    6    6                            tween back-                                                                   washing `t`                                                                   (hrs)                                                                         ______________________________________                                        Case (ii) Pressure increase with reverse flow of feed -                       Mode 2(a)                                                                     ______________________________________                                        Peak flux re-                                                                          1020   1010   960  840  820  820  820  820                           covery after                                                                  b/w(1/hr.m.sup.2)                                                             Flux after `t`                                                                          500    680   600  600  600  600  600  600                           hours of fil-                                                                 tration                                                                       (1/hr.m.sup.2)                                                                Interval be-                                                                            16      6     8    6    6    6    6    6                            tween back-                                                                   washing `t`                                                                   (hrs)                                                                         ______________________________________                                    

The flux decline in each case was fairly linear. Thus an average betweenstarting and finishing flux rates (in one time interval) was thought tobe a good basis on which to evaluate each of the above cases.

Reverse flow backwashing was clearly the best method of backwashing.After allowing transient flux increases to die away, an average filtrateflux of approximately 700 l/hr.m² was maintained.

After allowing transient flux increases to die away with normalbackwashing, an average filtrate flux of approximately 500 l/hr.m² wasmaintained.

Clearly the reverse flow backwash was about 40% more effective than anormal backwash.

EXAMPLE 10

A suspension of 600 g Ca(OH)₂ in 18.3 l water was filtered for 15minutes at 530 ° C. before application of each backwash mode as shown inTable VIII. The filtrate fluxes before and after backwash are shown inTable VIII.

The inlet pressure of the cartridge was 200 KPa(g), the outlet pressurewas 100 KPa(g) and the cross flow rate was about 2,500 litres/hour.

                  TABLE VIII                                                      ______________________________________                                                          Peak                    Peak                                BACKWASH  TIME    flux    BACKWASH  TIME  flux                                MODE      hr.min  1/hr    MODE      hr.min                                                                              1/hr                                ______________________________________                                                  0       1600              2.56  1400                                          .15      400              3.11   350                                1(b)                      4(b)                                                          .16     1400              3.12  1350                                          .31      410              3.27   330                                1(b)                      3(b)                                                          .32     1150              3.28  1450                                          .47      350              3.43   350                                1(b)                      3(b)                                                          .48     1100              3.44  1420                                          1.03     370              3.59   360                                1(b)                      3(b)                                                          1.04    1200              4.00  1350                                          1.19     370              4.15   320                                2(b)                      3(b)                                                          1.20    1400              4.16  1400                                          1.35     390              4.31   380                                2(b)                      5(b)                                                          1.36    1550              4.32  1400                                          1.51     420              4.47   380                                2(b)                      5(b)                                                          1.52    1450              4.48  1350                                          2.07     320              5.03   370                                2(b)                      5(b)                                                          2.08    1500              5.04  1330                                          2.23     340              5.19   350                                4(b)      2.24    1450              5.20  1400                                          2.39     320              5.35   350                                4(b)                      5(b)                                                          2.40    1400              5.36  1400                                          2.55     320                                                        4(b)                                                                                    5.51     320              7.16  1350                                3(a)                      5(a)      7.31   280                                          5.52    1300              7.32  1470                                          6.07     330              7.47   300                                3(a)                      5(a)                                                          6.08    1400              7.48  1450                                          6.23     310                                                        3(a)                                                                                    6.24    1350                                                                  6.39     340                                                        3(a)                                                                                    6.40    1300                                                                  6.55     310                                                        4(a)                                                                                    6.56    1300                                                                  6.11     300                                                        4(a)                                                                                    6.12    1300                                                                  6.27     320                                                        4(a)                                                                          6.28      1300                                                                          6.43     350                                                        4(a)                                                                                    6.44    1350                                                                  6.59     290                                                        5(a)                                                                                    7.00    1420                                                                  7.15     320                                                        5(a)                                                                          ______________________________________                                    

EXAMPLE 11

The same feedstock as Example 10 was filtered at 30° C. and backwashedafter 15 minutes in each case. Table IX shows the volume of backwashmaterial collected and the dry weight of the recovered solids.

                  TABLE IX                                                        ______________________________________                                                                           Recovered                                    Backwash  b/w material                                                                             Mass Ca(OH).sub.2                                                                         material                                     Mode      collected (1)                                                                            recovered (g)                                                                             g/l                                        ______________________________________                                        1(b)        6.3        281.9        44.7                                      7(h) (snd) feed off                                                                       2.54       232.9        91.7                                      7(b) (snd) feed off                                                                       1.82       202.4       111.2                                      7(d) (snd) feed off                                                                       2.54       168.7        66.4                                      7(g) (snd) feed off                                                                       1.84       173.9        94.5                                      7(a) (snd) feed off                                                                       1.47       148.5       101.0                                      7(c) (snd) feed off                                                                       1.67       126.8        75.9                                      7(g) (sd) feed off                                                                        0.12        42.8       356.7                                      7(a) (sd) feed off                                                                        0.27        51.0       188.9                                      7(c) (sd) feed off                                                                        0.13        28.0       215.4                                      7(g) (snd) air feed on                                                                    1.98       211.3       106.7                                      7(g) (sd) air feed on                                                                     0.56       159.6       285.0                                      7(e) (snd) air feed on                                                                    1.8        183.3       101.9                                      7(e) (sd) air feed on                                                                     0.24        95.4       397.7                                      7(a) (snd) air feed on                                                                    2.08       192.8        92.7                                      7(a) (sd) air feed on                                                                     0.44       118.7       269.8                                      7(c) (snd) air feed on                                                                    1.86       156.8        84.3                                      7(c) (sd) air feed on                                                                     0.35        80.73      230.7                                      7(i) (snd) air feed on                                                                    1.85       156.3        84.5                                      7(i) (sd) air feed on                                                                     0.43        81.1       188.7                                      ______________________________________                                    

EXAMPLE 12

In this experiment practical conditions limited the volume of feed,which was therefore recycled through the filter. Clots formed asmaterial was backwashed and blown back into the feed tank where theysettled to the bottom and no longer took part in the experiment. Thisresulted in a steady increase in the minimum flux value after a setinterval of time has elapsed after application of the backwash.

A tube in shell cartridge containing microporous polypropylene hollowfibres with approximately 1m² of filtering area was used to filter asuspension containing 50 g Bentonite and 50 g Diatomaceous Earth in 20litres of water. The suspension was applied to the outer surface of thefibres. The initial filtrate flux was 900 l/hr. After 10 minutes theflux had fallen to 200 l/hr.

A backwash Mode 1(b) was applied for 30 seconds. The 10 minute cycle wasrepeated 5 times, and each time the flux dropped to 200 l/hr to 250 l/hrbefore backwash, and rose to 600 l/hr after backwash. The point to whichthe flux dropped after each cycle was a little higher each time andclots of solid material could be observed in the material dischargedduring the backwash cycle. The results are graphed in FIG. 11.

Backwashes of Mode 2(b) were applied at the end of 10 minute cycles onthe same system. Each time the procedure was repeated the flux rose to900 l/hr.

At the conclusion of the Mode 2(b) series of backwash a backwash of Mode1(b) was again performed. The flux again rose to 600 l/hr after thebackwash cycle The results are graphed in FIG. 12.

FIG. 13 shows four cycles of reversed feedstock flows without anybackwashing followed by a backwash of Mode 2(b). The reversal ofdirection of flow by itself gives little improvement in flux. Thecombination of reversing the direction of flow, together with thebackwash Mode 1(b) gives a cleaning effect greater than the addition ofthe two separate techniques.

Various other modifications may be made to the filtration methods andcleaning cycles without departing from the scope and ambit of theinvention.

We claim:
 1. A method of operating a filter having elastic, microporous,hollow fibers within a shell or housing having a feed inlet thereto anda feed outlet therefrom comprising the steps of:(i) introducing a liquidsuspension feedstock through the feed inlet to the shell or housing anddirecting said feedstock to the outer surface of the fibersincluding:(a) passing some of said feedstock through the walls of thefibers so as to be drawn from the fiber lumens as a filtrate orpermeate, (b) retaining some of the solids in said feedstock on or inthe pores of the fibers, and discharging the non-retained solids througha feed outlet from the shell or housing with a remainder of saidfeedstock, (ii) periodically cleaning away the retained solids by:(a)introducing a pressurized gas into the fiber lumens which passes throughthe walls of the fibers to dislodge the retained solids, and, (b)varying the pressure within the shell while the gas is being introducedinto the lumens.
 2. A method of operating a filter having a plurality ofelastic, microporous hollow fibers with a shell or housing having a feedinlet thereto and a feed outlet therefrom, comprising the steps of:(i)introducing a liquid suspension feedstock into the shell or housing andapplying said feedstock to the outer surface of the fibers including:(a)passing some of said feedstock through the walls of the fibers so as tobe drawn from the fiber lumens as a filtrate or permeate, (b) retainingsome of the solids in said feedstock on or in the pores of the fibers,and removing the non-retained solids from the shell with a remainder ofsaid feedstock, (ii) cleaning away the retained solids by:(a) applyingto the fiber lumens a gas at a pressure sufficient to stretchsubstantially all of the pores, followed by, (b) maintaining the flow ofgas through the lumens for a time period sufficient to drive the liquidfrom the pores having a bubble point below the pressure of the gas so asto wash out any solids in those pores and to dislodge any solidsretained on the outer surface of the fibers such that the washed anddislodged solids are removed from the shell to an external collectionpoint, and, (c) varying the pressure within the shell whilst thepressurized gas is being introduced into the lumens.
 3. A methodaccording to claim 1 or claim 2 wherein the pressure within the shell isvaried during cleaning by increasing the pressure within the shell abovethe normal gaseous cleaning pressure and then returning the pressure tothe normal gaseous cleaning pressure.
 4. A method according to claim 3wherein the feed flow is maintained during the backwash and the pressureincrease is achieved by stopping the outflow of feed and the return tothe normal gaseous cleaning pressure is achieved by recommencing flow offeed in the reverse direction.
 5. A method according to claim 4 whereinthe reversal of feed flow is repeated during gaseous cleaning.
 6. Amethod according to claim 3 wherein the feed flow is maintained duringthe backwash and the pressure increase is achieved by terminating theoutflow of feed and the return to normal gaseous cleaning pressure isachieved by resuming outflow of feed.
 7. A method according to claim 6wherein the termination and resumption of feed outflow is repeatedduring gaseous cleaning.
 8. A method according to claim 3 wherein thepressure variation cycle is repeated during gaseous cleaning.
 9. Amethod according to claim 3 wherein the inflow of feed is terminatedbefore commencing the gaseous backwash.
 10. A method according to claim9 wherein the shell outlet is closed and further including the stepsof:(a) introducing a second gas into the shell side of the fibers at apressure substantially the same as the pressure of the lumen cleaninggas, (b) terminating the flow of the second gas, (c) opening one of thefeed inlet and the feed outlet to release the gas pressure on the shellside of the fibers and to allow the lumen gas to escape substantiallyuniformly through the fiber walls.
 11. A method according to claim 9 andincluding the step of introducing a second gas through the shell inletand wherein the pressure increase is achieved by stopping the outflow ofthe second gas and the return to the normal gaseous cleaning pressure isachieved by recommencing the flow of the second gas in the reversedirection.
 12. A method according to claim 11 wherein termination andreversal of the second gas flow is repeated during gaseous cleaning. 13.A method according to claim 12 wherein the second gas is a high pressuregas.
 14. A method according to claim 11 wherein the second gas is a highpressure gas.
 15. A method according to claim 14 wherein the second gasis a low pressure gas.
 16. A method according to claim 11 wherein thesecond gas is a low pressure gas.
 17. A method according to claim 9 andincluding the step of introducing a second gas through the shell inletand wherein the pressure increase is achieved by terminating the outflowof the second gas and the return to normal gaseous cleaning pressure isachieved by resuming the outflow of the second gas.
 18. A methodaccording to claim 17 wherein the termination and resumption of secondgas outflow is repeated during gaseous cleaning.
 19. A method accordingto claim 18 wherein the second gas is a high pressure gas.
 20. A methodaccording to claim 18 wherein the second gas is a low pressure gas. 21.A method according to claim 17 wherein the second gas is a high pressuregas.
 22. A method according to claim 17 wherein the second gas is a lowpressure gas.
 23. A method according to claim 22 and including the stepof introducing a second gas through the shell inlet and wherein thepressure reduction is achieved by terminating the inflow of the secondgas and the return to normal gaseous cleaning pressure is achieved byrecommencing the flow of the second gas in the reverse direction.
 24. Amethod according to claim 23, wherein the termination and reversal ofthe second gas flow is repeated during gaseous cleaning.
 25. A methodaccording to claim 24 wherein the second gas is a high pressure gas. 26.A method according to claim 24 wherein the second gas is a low pressuregas.
 27. A method according to claim 23 wherein the second gas is a highpressure gas.
 28. A method according to claim 23 wherein the second gasis a low pressure gas.
 29. A method according to claim 1 or claim 2wherein the pressure within the shell is varied during cleaning bydecreasing the pressure within the shell below the normal gaseouscleaning pressure and then returning the pressure to the normal gaseouscleaning pressure.
 30. A method according to claim 29 wherein the feedflow is maintained during the backwash and the pressure reduction isachieved by terminating the inflow of feed and the return to normalgaseous cleaning pressure is achieved by resuming inflow of feed.
 31. Amethod according to claim 30, wherein the termination and resumption offeed inflow is repeated during gaseous cleaning.
 32. A method accordingto claim 24 wherein the pressure variation cycle is repeated duringgaseous cleaning.
 33. A method according to claim 29 wherein the feedflow is maintained during the backwash and the pressure reduction isachieved by terminating the inflow of feed and the return to normalgaseous cleaning pressure is achieved by recommencing flow of feed inthe reverse direction.
 34. A method according to claim 33 wherein thetermination and resumption of feed is repeated during gaseous cleaning.35. A method according to claim 29 wherein the inflow of feed isterminated before commencing the gaseous backwash.
 36. A methodaccording to claim 35 and including the step of introducing a second gasthrough the shell inlet and wherein the pressure reduction is achievedby terminating the inflow of the second gas and the return to normalgaseous cleaning pressure is achieved by recommencing flow of the secondgas.
 37. A method according to claim 36 wherein the second gas is a highpressure gas.
 38. A method according to claim 36 wherein the terminationand resumption of outflow is repeated during gaseous cleaning.
 39. Amethod according to claim 38 wherein the second gas is a low pressuregas.
 40. A method according to claim 38 wherein the second gas is a highpressure gas.
 41. A method according to claim 38 wherein the second gasis a low pressure gas.
 42. A method according to claim 35 wherein theshell outlet is closed and further including the steps of:(a)introducing a second gas into the shell side of the fibers at a pressuresubstantially the same as the pressure of the lumen cleaning gas, (b)terminating the flow of the second gas, (c) opening one of the feedinlet and the feed outlet to release the gas on the shell side of thefibers and to allow the lumen gas to escape substantially uniformlythrough the fiber walls.
 43. A method according to claim 1 and includingthe step of pressurizing the fibers after the completion of the backwashand the releasing the pressure to remove trapped air from the pores ofthe fibers.
 44. A method according to claim 43 wherein the step ofpressurizing the fibers is carried out while lumen flow is blocked. 45.A method according to claim 44 wherein the lumen flow is blocked in apulsing fashion.
 46. A method according to claim 45 wherein the step ofpressurizing the fibers is carried by applying a hydraulic pressure tothe feed side of the fibers.
 47. A method according to claim 45 whereinthe step of pressurizing the fibers is carried out by applying ahydraulic pressure to the feed side of the fibers.
 48. A methodaccording to claim 43 wherein the step of pressurizing the fibers iscarried out by applying a hydraulic pressure to the feed side of thefibers.
 49. A method according to claim 43 wherein the step ofpressurizing the fibers is carried out by applying a hydraulic pressureto the feed side of the fibers.
 50. A method according to claim 1wherein the fiber lumens are drained before commencement of the gaseousbackwash.
 51. A method according to claim 1 wherein the shell is drainedbefore commencement of backwash.
 52. A method according to claim 1wherein the introduction of the pressurized gas includes the stepsof:(a) initially applying the gas at a pressure below the bubble pointof the walls of the fibers so as to displace any liquid from the fiberlumens, (b) closing the feed inlet to and the feed outlet from theshell, (c) increasing the pressure of the gas above the bubble point ofthe walls of the fibers, and, (d) opening the feed inlet and outlet toallow the gas to escape substantially the uniformly through the fiberwalls.
 53. A method according to claim 1 wherein the filter is across-flow filter.
 54. A method according to claim 1 wherein the filteris operated in a dead-end filtering mode with no outflow of feed andsolids from the shell during the dead-end filtration.
 55. A methodaccording to claim 1 wherein during the pressure variation step, feed isintroduced into the shell between the shell inlet and outlet and isdischarged through both the shell inlet and shell outlet to an externalcollection point.
 56. A filter system comprising:(a) a shell or housing,(b) a plurality of elastic, microporous hollow fibers within the shell,(c) a feed inlet to the shell, (d) a feed outlet from the shell, (e) afiltrate outlet from the shell, (f) valve means for introducing a liquidsuspension feedstock through the feed inlet to the shell and fordirecting said feedstock to the outer surface of the fibers suchthat:(i) some of said feedstock passes through the walls of the fibersto be drawn from the fiber lumens as a filtrate or permeate and to bedischarged through the filtrate outlet, (ii) some of the solids in saidfeedstock are retained on or in the pores of the fibers, with thenon-retained solids being discharged through the feed outlet from theshell or housing with the remainder of said feedstock, (g) valve meansfor controlling the outflow through the shell outlet, (h) valve meansfor introducing a pressurized gas into the fiber lumens which passesthrough the walls of the fibers to dislodge the retained solids, and,(i) control means for varying the pressure within the shell while thegas is being introduced into the lumens.
 57. A filter systemcomprising:(a) a shell or housing, (b) a plurality of elastic,microporous hollow fibers within the shell, (c) a feed inlet to theshell, (d) a feed outlet from the shell, (e) a filtrate outlet from theshell, (f) valve means for introducing a liquid suspension feedstockthrough the feed inlet to the shell and for applying said feedstock tothe outer surface of the fibers such that:(i) some of said feedstockpasses through the walls of the fibers to be drawn from the fiber lumensas a filtrate or permeate and to be discharged through the filtrateoutlet, (ii) some of the solids in said feedstock are retained on or inthe pores of the fibers, with the non-retained solids being removed fromthe shell with the remainder of said feedstock, (g) valve means forcontrolling the outflow through the shell outlet, (h) valve means forintroducing a pressurized gas through the fiber lumens which passesthrough the walls of the fibers to wash out at least some of theretained solids and then, (i) valve means for introducing through thefiber lumens a pressurized gas which passes through the walls of thefibers and stretches elastically at least some of the pores to dislodgeany solids retained in those pores and which washes the external wallsof the fibers, the gas being applied at a pressure which is sufficientto overcome the resistance to gas flow of the surface tension of thecontinuous phase of the control filtrate within the pores of themembrane, and, (j) means for varying the pressure within the shellwhilst the pressurized gas is being introduced into the lumens.
 58. Afilter system according to claim 56 or claim 57 wherein the controlmeans is adapted to actuate the valve means to increase the pressurewithin the shell above the normal gaseous cleaning pressure and toreturn the pressure to the normal gaseous cleaning pressure.
 59. Afilter system according to claim 58 wherein the control means is adaptedto actuate the shell outlet valve means to close the shell outlet andthen to open the shell outlet.
 60. A filter system according to claim 59wherein the control means is adapted to actuate the shell outlet valvemeans to close the shell outlet and wherein the system further includesvalve means for reversing the flow of feed through the shell and thecontrol means is adapted to actuate the reversing valve means tore-establish reverse flow of feed through the shell.
 61. A filter systemaccording to claim 59 wherein the control means is adapted to actuatethe shell inlet valve means to close the shell inlet and wherein thesystem further includes valve means for reversing the flow of feedthrough the shell and the control means is adapted to actuate thereversing valve means to reestablish reverse flow of feed through theshell.
 62. A filter system according to claim 56 or claim 57 wherein thecontrol means is adapted to actuate the valve means to decrease thepressure within the shell below the normal gaseous cleaning pressure andto return the pressure to the normal gaseous cleaning pressure.
 63. Afilter system according to claim 62 wherein the control means is adaptedto actuate the shell inlet valve means to close the shell inlet valveand to open the shell inlet.
 64. A filter system as claimed in eitherclaim 56 or claim 57 and including valve means for introducing a secondgas through the shell inlet.
 65. A filter system according to claim 64wherein the control means is adapted to close the shell outlet to effecta pressure increase in the shell and to open the shell outlet tore-establish outflow through the shell outlet.
 66. A filter system,according to claim 65 and further including valve means for reversingthe flow of the second gas through the shell and wherein the controlmeans is adapted to operate the reversing valve means.
 67. A filtersystem according to claim 66 and further including valve means forreversing the flow of the second gas through the shell and wherein thecontrol means is adapted to operate the reversing valve means.
 68. Afilter system according to 66 and including means for closing off thefiltrate flow.
 69. A filter system according to claim 64 wherein thecontrol means is adapted to close the shell inlet to effect a pressuredecrease in the shell and to open the shell inlet to re-establish inflowof the second gas.
 70. A filter system according to claim 64 andincluding means for closing off the filtrate flow.